The invention relates to a drive-train control system for a motor vehicle which an engine and automatic transmission.
A drive-train control system of this kind includes an engine and an engine control system, and is used to control variables that influence engine torque, e.g. an ignition point and fuel metered to the cylinders. It also includes an automatic transmission and a transmission control system, which is used to control shift operations of the automatic transmission. The transmission shift points used by the transmission control system are stored in one or more map memories (e.g. International Patent Disclosure WO 91/13780).
In almost all known automatic transmission control systems, the respective gear to be selected is chosen by shift characteristics, which are stored in a map memory in the form of a function of or dependence on the vehicle speed and the position of the accelerator pedal or of the throttle valve. In defining the shift characteristics, it is assumed that there is a fixed correlation between a particular position of the accelerator pedal or throttle valve and a particular operating point of the engine. This applies both to the torque output and to the fuel consumption incurred in the process.
However, this assumption leads to difficulties if the position of the accelerator pedal or of the throttle valve does not bear an unambiguous relationship to the engine torque. This is the case, for example, when an engine has different operating modes. In the case of direct gas injection, for example, the power or torque during lean-mixture operation is determined by way of the injection time, with the throttle valve being fully open. In the case of normal operation at xcex=1, on the other hand, the power and torque are adjusted by the throttle valve angle. In the case of diesel engines, there is no throttle valve at all and, in this case, the torque is adjusted by the injection time alone. Another example of the absence of a correlation between the engine torque output and an operating parameter of the engine is the position of the accelerator pedal in the case of an electronically controlled throttle valve (ETC). With a throttle valve of this kind, it is possible to implement a large number of functions, in which a single position of the accelerator pedal leads to different engine torques depending on the operating state. At low vehicle speeds and a small deflection of the accelerator pedal, for instance, a lower sensitivity is specified in order to make maneuvering and parking easier. In the case of shift operations in the transmission too, the engine torque is modified independently of the position of the accelerator pedal in order to stabilize the wheel torque.
Another known drive-train control system of a motor vehicle with an engine and an automatic transmission (see Japanese Preliminary Published Patent Application JP 7-89373 together with Japanese Abstract) is provided with an engine control system, by which variables that influence the engine torque are controlled, with a transmission control system, by which the shift operations of the automatic transmission are controlled, and with a map memory, in which transmission shift points used by the transmission control system are stored as a function of the vehicle speed and the engine torque. The map memory does not exchange any information with the engine control system. In particular, the transmission control system does not receive any information on the maximum engine torque and the maximum available torque reserve. The map memory is supplied only with information from sensors or detectors for the engine torque and the vehicle speed. The above-mentioned cases lead to maladjusted shift behavior if the engine characteristics assumed for the definition of the shift characteristics, i.e. a predetermined constant engine behavior, change in reality.
It is accordingly an object of the invention to provide a drive-train control system of a motor vehicle which overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which the engine behavior is separated from the shift characteristics, i.e. to allow definition of the shift characteristics for the transmission in a manner which is largely independent of the engine characteristics.
With the foregoing and other objects in view there is provided, in accordance with the invention, a combination of a motor vehicle having an engine and an automatic transmission, with a drive-train control system. The drive-train control system has an engine control system for controlling variables that influence an engine torque and a transmission control system for controlling a ratio of the automatic transmission, the transmission control system being connected to the engine control system. A map memory is connected to the transmission control system and contains a shift map having characteristics defined in a vehicle-dependent manner taking into account a maximum possible engine torque. The transmission control system selects the ratio to be set from the shift map in dependence on a torque demanded by a driver and on a variable functionally linked to a speed of the motor vehicle.
The drive-train control system of the motor vehicle having an engine and an automatic transmission is provided with an engine control system by which variables that influence the engine torque are controlled, with a transmission control system by which the ratio of the automatic transmission is controlled, and with a map memory. From the map memory a ratio to be set by the transmission control system can be taken in dependence on the torque demanded by the driver and, on a variable functionally linked to the vehicle speed, and the characteristics of which are defined in a vehicle-dependent manner taking into account the maximum possible torque.
Some advantages of the invention are that, in conjunction with the choice of gear, it allows typical parameters for the engine torque produced to be found on the basis of the shift characteristics defined in accordance with the invention. The engine characteristics can be transmitted with significant variables from the engine control system to the transmission control system in compressed form, i.e. with very few data, so as to decouple the two systems as far as possible. The shift characteristics are defined in a manner which is as independent as possible from the engine and do not contain a torque model of the engine.
The engine characteristics are concentrated at the interface between the transmission and the engine. Therefore, the shift characteristics can be defined to a large extent independently of the engine. This eliminates the need to adapt them individually to different types of engine. Given the large number of options for a vehicle that are available today and the expectation of even more in the futurexe2x80x94the so-called single platform with different engine and transmission optionsxe2x80x94leads to a considerable reduction in the development work required for the drive-train control system. Moreover, the invention allows consistently torque-based control of the drive train, which will gain widespread acceptance with the increasing introduction of electronically controlled throttle valves and the so-called torque interface. The interface between the engine and the transmission can thus be defined in such a way that specific characteristics of the components will in each case only be present in the system concerned.
As mentioned, there are cases where an engine has different characteristics depending on the operating mode (e.g. in the case of direct gas injection). In the case of conventional transmission control systems, this leads to complicated solutions, leading in extreme cases to a doubling in the number of shift maps. In the case of adaptive transmission control systems, up to 10 different shift maps are required. This complexity is reduced in an effective manner by the invention.
The subjective impression of the driver, i.e. the impression that the driver gets of the behavior and performance capabilities of the drive train, is essentially determined by the torque reserve. In this context, the driver gets a feeling for how much power the vehicle still has to spare at any given time (before the maximum available power output is reached) for accelerating the vehicle or climbing a slope at a uniform speed, for example. The invention is particularly advantageous because the subjective impression of the driver can be transferred to different vehicles with different engine options without any outlay.
Moreover, special operating modes of the engine (e.g. a decidedly economical mode), which result in special engine characteristics, can be readily taken into account. If a reduced maximum engine torque is available, the downshifts take place earlier without the need for a special set of characteristics for this purpose.
In accordance with an added feature of the invention, the shift map contains the ratio of the automatic transmission as a function of available torque reserve and an output speed of the automatic transmission.
In accordance with an additional feature of the invention, the shift map contains the ratio of the automatic transmission as a function of available torque reserve and the speed of the vehicle.
In accordance with another feature of the invention, the shift map contains shift points in a form of a function of a torque reserve available at a driven wheel and engine speed.
In accordance with a further feature of the invention, the shift map contains shift points as a function of a normalized engine torque.
In accordance with another added feature of the invention, the normalized engine torque is defined as a quotient of a current engine torque and the maximum possible engine torque.
In accordance with concomitant feature of the invention, the shift map is one of a plurality of shift maps containing the ratio of the automatic transmission, and an adaptation circuit is connected to the map memory. The adaptation circuit chooses one of the shift maps as a function of a driving state to determine the ratio.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a drive-train control system of a motor vehicle, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.